Study On The Process Of River Scouring-silting And Depletion Of Oxygen During Rainfall Period

Dissolved Oxygen(DO)is one of the important indicators to measure the water quality of water bodies.Aquatic animals generally live in water with DO>5 mg/L.Aquatic organisms can hardly survive and,even the water body will appear black and smelly When DO ≤ 2 mg/L,at this time,it is considered that the river is deficient in oxygen.When the DO is 0 mg/L,it is absolutely deficient in oxygen.Under normal circumstances,the loose silt sinks at the bottom and will not migrate with the flow,the water flow is only the upper water body.During the rainfall period,as the rainwater and surface runoff merge into the river course,those gradually converge to form a flood peak,and the bottom shear force increases.The flow rate of the river water body speeds up.When the water flow velocity is greater than the initial flow rate of the silt,the sludge starts to scour with the flow,and the organic sludge is entrained into the water body,and the rainwater entrains the silt and transports it downstream,thus forming a flushing process.The sludge is entrain into the water body,the concentration of organic matter in the water rises sharply,and the organic matter quickly consumes the dissolved oxygen in the water body,resulting in a rapid decrease of dissolved oxygen in the water body,and even an absolute oxygen deficient state with dissolved oxygen of 0 mg/L,which is seriously endangered for the survival of fish and other aquatic animals in the watershed and destroys the diversity of the water ecological environment;at the same time,the turbulence in the watershed during the rainfall period increases,accelerating the entry of atmospheric oxygen into the water body,replenishing the dissolved oxygen in the water body.Further the rainwater and the direct runoff full of saturated dissolved oxygen,will directly flows into the river to supplement the dissolved oxygen in the water body.The "Shuanghekou Section-Tanyansuo Section" in the small river basin of the Fuxi River has a small slope and a relatively flat flow rate.In addition to the long-term input of pollutants from surrounding industries,agriculture,livestock and poultry breeding,and domestic wastewater,silt with high content of organic matter is formed at the bottom of the river.This is the most heavily silted section in the Fuxi river basin.This paper uses this siltated river section as the research background,by constructing a one-dimensional transport model of scouring and oxygen during the rainfall period,to study the process of scouring and oxygen depletion in the siltated river section during the rainfall period.The main results are as follows:(1)The local rainfall data indicated that the rainfall in the basin was unevenly distributed between years and during the year.From 2015 to 2017,the annual rainfall in 2015 was the highest with 1,056.60mm;the year with the lowest annual rainfall was 2017 with only597.05 mm.The distribution of rainfall during the year changed seasonally.Rainfall was mainly concentrated in from May to October.The average monthly rainfall was about106.29mm;the rainfall from November to April was scarce,and the rainfall in January and February of 2017 was only 5mm.(2)Based on the main stream length,river flow,flow rate,gradient and other basic data of the Fuxi River Basin,as well as basic data such as topography,water system,and meteorology,analyzed the temporal and spatial distribution of rainfall and runoff characteristics;calculated the river flow during the rainfall period by formula method,Hydrodynamic characteristics such as flow velocity and transport time.(3)According to the actual rainfall data,we compared the total scouring amount with the siltation reduction by calculating the scouring model.The results showed that the amount of erosion in 2015 and 2016 was basically consistent with the decrease in siltation with the amount of erosion accounted for 76.64% and 84.87% of the decrease in siltation,respectively.In 2017,the amount of erosion only accounted for 26.68% of the decrease in siltation.(4)Based on the research background of the siltated section of the Fuxi River,a onedimensional unsteady water quality model of scouring and oxygen transport was established to explore the short-term changes of the channel scouring and dissolved oxygen during the rainfall period.The results showed that: the greater the rainfall intensity,the closer to the downstream will deliver,the higher COD concentration in the river water body,and the smaller the DO concentration makes,dissolved oxygen below 2 mg/L easier for the oxygen-deficient,and even the absolute oxygen deficit reaches for 0 mg/L may appear.